Metallic structures located in soil, water or concrete are always subjected to more or less corrosion. If the construction in addition is subjected to stray currents or another external influence, such as a chloride-containing environment, the corrosion is usually increased. In these cases, the corrosion arises not only in the form of general corrosion but also in the form of localized corrosion, usually pitting, which gives a considerably faster penetration of the construction than general corrosion.
Steel embedded in concrete is generally protected from corrosion by the high alkalinity of the concrete, whereby the steel surface becomes passivated. However, by increased content of chloride of the pore water of the concrete and/or carbonation, which arises because of lowering of the pH value of the concrete as a consequence of the influence of the carbon dioxide of the air, the passivity of the steel may be disrupted and corrosion initiated. Already after a relatively small corrosion attack, cracks arise and then spalling of concrete due to the voluminous corrosion products of the steel. Today, carbonation is a smaller problem, but chloride-initiated corrosion is present to a great extent. Chloride-initiated corrosion is above all a common problem in concrete structures arranged in a marine environment or road environments.
In spite of extensive research, it has not been possible to establish reliable chloride threshold values of when the corrosion is initiated on steel embedded in concrete. Other circumstances such as concrete composition, moisture content, presence of different types of corrosion cells, etc., have influence. This entails great difficulties in condition assessments and thereby to know when, for instance, repair measures should be applied. Measures applied too late may cause security risks and increased repair costs.
Pipe conduits installed in the ground are often buried in the vicinity of power lines in order to in this way utilize the easements that already are there. This makes among other things that stray currents are often found in the vicinity of the pipe conduits.
Pipe conduits installed in the ground and made of steel for oil and gas are usually protected against corrosion by an outer protective coating in combination with a cathodic protection. The protective effect can be checked by a so-called potential measurement. However, in areas of stray currents, there are difficulties to maintain a reliable protection due to strong local variations. Accordingly, in these areas, there is a need of monitoring the corrosion that after all takes place.
Today, modern pipe conduits are provided with an outer coating of thick polyethylene in order to protect against corrosion attacks. However, small mechanical defects inevitably arise in the coating. By this protective coating, the pipe is also insulated against ground, and as a consequence of this, high alternating current potentials may be built up in the pipe conduit due to induction from adjacent power lines. In case of a high alternating current potential on the pipe conduit, alternating currents of a high current density may flow between the pipe and the surrounding soil in coating defects. In case of high alternating current densities, alternating current corrosion arises. This has resulted in serious localized corrosion with perforations as a consequence. Hence, the ambition to minimize the corrosion of pipe conduits by providing them with a protective coating has created a new corrosion phenomenon. In spite of intense research, no reliable criteria have been possible to be established. In certain cases, corrosion arises, in other not.
In order to monitor corrosion of buried pipe conduits, today usually two different techniques are used. The first technique utilizes test plates, which is based on the fact that test plates, weighed in advance, are buried adjacent to the pipe and electrically connected to the pipe. After a certain predetermined time, the plates are dug up and the occurred general and localized corrosion, respectively, of the test plates are evaluated by measuring pit depth and evaluation of mass change, respectively. The technique of employing test plates has, on one hand, economical disadvantages as a consequence of high costs of burying and digging up as well as evaluation, and information about serious corrosion is, on the other hand, often obtained far later than when it has occurred.
The second technique utilizes so-called ER (Electrical Resistance)-probes. ER-probes are based on the principle that the resistance in a wire or a sheet of the metal increases when the amount of metal decreases as a consequence of corrosion. In this case, the rate of corrosion can be measured continuously. ER-probes provide good information about the magnitude of the general corrosion, however no or at least only a very small indication of local corrosion attacks is obtained, such as pitting, since they only marginally have an impact on the total resistance of the wire/sheet. For a pipe conduit, it is the pitting corrosion rate that is the most important parameter, since a local pit may give rise to leakage.
In case of stray current influence, particularly from alternating current, there is a tendency that corrosion attacks become of local character i.e., local pits. As mentioned above, these cannot in a reliable way be recorded by ER-probes. Neither is the technique of employing test plates sufficient and it is expensive, as has been described above.
JP 2107947 discloses a solution to measure corrosion on a metallic buried pipe. A box body comprising closed hollow spaces is buried in the vicinity of the pipe. The box body has a wall thickness that is less than the wall thickness of the metallic pipe. The closed hollow spaces are provided with a pressurized medium having a predetermined pressure via connected conduits. The pressure of the medium is measured by a pressure gauge. When the wall of the box body has corroded so much that a hole has been formed in the wall, the pressure in the closed hollow space will fall, which is detected by means of the pressure gauge. By detecting a reduction of the pressure, information is thus obtained about the wall of the box body having corroded so that the wall has been perforated, and by that the corrosion has reached a critical level.
However, the solution disclosed in JP 2107947 has the disadvantage that it does not reflect the real conditions that the metallic pipe conduit usually is subjected to. According to the solution disclosed in the document, the box body will not be electrically insulated and will accordingly not be subjected to alternating current influence and high current densities in the same way as a metallic pipe having a protective coating.
For protective coated metallic structures that are not cathode protected, in certain cases also moisture may migrate in, for instance, from the soil via a local defect in the protective coating, between the protective coating and the metallic structure, whereby a local corrosion cell may be formed under the coating. This local corrosion cell may give rise to such corrosion that the coating risks peeling off from the surface of the metallic pipe.